Quality and Significance of Irrigation Return Flow

A study of irrigation return flow in the Yakima River Basin showed that return flow was the major factor influencing Yakima River water quality. The increase in the quantity and composition of salts in the return flow is due more to leaching than to evapo-transpiration. Water quality changes occurring during irrigation are examined together with techniques and objectives for return flow quality studies. Data are presented on water and salt budgets together with the ion concentrations in the applied water, the subsurface, and the surface drainage water. Salts leached and exchanged from the soil and released to the parent river in the irrigation and nonirrigation seasons are reported on a weight-per-acre basis. The impact of domestic sewage and industrial effluents on river chemical quality is shown to be but a small fraction of that due to return flow. Sixty-one percent of the lower river salt content by weight was contributed by return flow in the irrigation season and 49% in the nonirrigation season.